Medical ventilators

ABSTRACT

A medical ventilator comprises a ventilator duct having an inlet for respiratory fresh gas. The pressure at which the respiratory fresh gas is supplied to the patient is determined by a driving jet disposed in the ventilator duct at a distance from the inlet so that an intervening column of gas acts as a pneumatic piston. The supply of driving gas to the jet is initially set by a regulator to obtain a desired value of a respiratory parameter, such as the volume of gas ventilating the patient&#39;s lungs during the breathing cycle. The volume of gas expelled from the patient&#39;s lungs is monitored during operation by detecting means and the regulator is adjusted by a servomotor in response to any change in the measured parameter in order to maintain the parameter at the desired value.

This invention relates to improvements in or relating to medicalventilators and, in particular, concerns improvements in the control ofthe operation of medical ventilators.

There are two basic types of known medical ventilators.

A first known kind of medical ventilator, termed a constant volume flowgenerator, operates with respiratory fresh gas at relatively highpressure and therefore functions as a flow generator capable ofproviding a set flow of respiratory fresh gas such that the volume ofrespiratory gas supplied to a patient during a breathing cycle remainsapproximately constant irrespective of changes in the pulmonarycharacteristics, i.e. resistance of the airways and pulmonarycompliance, which constitute a load to the ventilator. This type ofventilator is not versatile for all age groups of patients and suffersfrom high lung pressures.

A second known kind of medical ventilator operates with a relatively lowgenerated pressure and functions as a pressure generator which is notcapable of maintaining constant the volume of the ventilatingrespiratory gas supplied to the patient during a breathing cycle in theface of changes in the pulmonary characteristics. This pressuregenerator type of ventilator is highly versatile and can be used safelyfor any patient age groups.

Because known medical ventilators of the second kind are preset tomaintain pressure conditions which are constant once set, changes in thecondition of the lungs of a patient connected to such a ventilator,after the initial pressure conditions have been set to suite the initialcondition of the patient's lungs, can cause considerable changes inphysiological respiratory parameters, such as the volume of gasdelivered to the patient's lungs during the breathing cycle or theconcentration of carbon dioxide in the expired gas at the end of theexpiration phase of the breathing cycle, the so-called end tidal carbondioxide concentration.

It is an object of the present invention to provide an improved medicalventilator of the pressure generator kind and a method of operating sucha ventilator to regulate automatically a selected physiologicalparameter of the patient's respiration, even though the pulmonarycharacteristics change over time.

Accordingly, in one aspect, the present invention provides a medicalventilator comprising means for initially setting the pressure ofrespiratory fresh gas supplied to a patient to obtain a desired value ofa respiratory parameter to be controlled, means for measuring the valueof the respiratory parameter during subsequent operation of theventilator, means for detecting in the measured value of the respiratoryparameter any change from the desired value, and means for adjusting thesetting means in dependent upon the change in the measured value of therespiratory parameter in order to vary the pressure of respiratory freshgas supplied by the ventilator so as to maintain the respiratoryparameter at the desired value.

In another aspect, the invention provides a method of operating amedical ventilator in which the pressure of respiratory fresh gassupplied to a patient is variable, comprising initially setting thepressure of the respiratory fresh gas to obtain a desired value of arespiratory parameter to be controlled, measuring the respiratoryparameter during subsequent operation of the ventilator, detecting inthe measured respiratory parameter any change from the desired value,and adjusting the pressure of the respiratory fresh gas supplied by theventilator in dependence upon the change in the respiratory parameter soas to maintain the respiratory parameter at the desired value.

In order that the invention may be more readily understood, embodimentsthereof will now be described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic representation of one medical ventilatorembodying the present invention; and

FIG. 2 is a diagrammatic representation of another medical ventilatorembodying the invention.

Referring firstly to FIG. 1, one embodiment of a medical ventilatoraccording to the present invention comprises a ventilator duct 1 havinga flexible patient tube 2 by which the duct 1 is connected to a trachealtube 3 for insertion into the airway of a patient 4.

A source 5 of respiratory gas at relatively low pressure is connected bya respiratory fresh gas conduit 6, which is relatively narrow comparedto the ventilator duct, to an inlet 7 at the patient end of the patienttube 2 adjacent the proximal end of the tracheal tube 3. A continuousflow of respiratory gas is supplied through the conduit 6 duringoperation of the ventilator and a flow sensor 8 in the fresh gas conduit6 is arranged to measure the volume of respiratory fresh gas suppliedthrough the conduit 6 during the expiratory phase of each breathingcycle.

A driving gas jet 10 is located in the ventilator duct 1 and isconnected to a source 11 of high pressure driving gas, which may or maynot be the same as the respiratory fresh gas by a supply line 12. Asolenoid valve 14 is incorporated in the supply line 12 and iscontrolled by an electrical pulse generator (not shown) to act as achopper to supply pulses of high pressure gas to the jet 10 which isorientated to deliver the driving gas axially into the ventilator duct 1towards the patient end thereof, so as to impose a cyclically varyingpressure on the low pressure respiratory gas delivered at inlet 7through the fresh gas conduit 6. The distance between the inlet 7 andthe jet 10 is such that the column of gas in the ventilator duct betweenthe jet 10 and the inlet 7 acts as a pneumatic piston to impose therequired pressure variation on the respiratory fresh gas delivered atinlet 7 without the driving gas itself taking part in the respirationprocess.

The gas emerging from the patient's lungs during the expiration phase ofthe breathing cycle passes out of the ventilator, together with thefresh gas supplied to inlet 7 during the expiration phase, through therelatively wide ventilator duct 1. A flow sensor 15 is incorporated inthe ventilator duct and is arranged to measure the total expired volumecomposed of the volume of gas expelled from the patient's lungs and thevolume of respiratory fresh gas supplied during the expiration phase.

A regulator 16, which may be either a flow regulator or a pressureregulator, is connected in the supply pipe 12 between the solenoid valve14 and the driving gas source 11 and has a control member 17 which isadjustable manually to vary either the flow rate or the pressure of thegas supplied to the solenoid valve 14. The flow sensors 8 and 15 deliverrespective electrical measurement signals which are significant of thevolume of fresh gas supplied through the fresh gas supply duct 6 duringthe expiration phase of breathing in the case of sensor 8 and of thetotal volume of gas passing through the ventilator duct 1 during theexpiration phase in the case of sensor 15. The electrical measurementsignals are supplied on lines 18 and 19 to a subtracting circuit 20which establishes the difference between the signals delivered by thetwo sensors 8 and 15. The resultant electrical measurement signaldelivered at the output 21 of the subtracting circuit 20 is significantof the volume of gas expelled from the patient's lungs during theexpiration phase of breathing and corresponds to the volume of gasventilating the patient's lungs, i.e. the so-called expired tidalvolume. The signal at the output 21 of circuit 20 is delivered to afirst input 22 of a comparator 23 via a reference memory 24 which isconnected to the output 21 by a normally closed isolating switch 25(shown however in its open condition). The output 21 of the comparator20 is also connected to a second input 26 of the comparator 23 via acurrent value memory 27. An output 28 of the comparator 23 is connectedto a servo motor 29 which acts to adjust the control member 17 of theregulator 16. The output 21 of the subtracting circuit 20 may also beconnected to a display device 30 for displaying the measured value ofthe ventilating volume.

In use of the ventilator described above with reference to FIG. 1, thecontrol member 17 of the regulator 16 is initially manually adjusted atthe start of ventilation to deliver a desired value of the volume of gasto the patient's lungs and which it is desired to maintain for thepatient under ventilation regardless of changes in the patient'scondition. Once the desired value of the ventilating volume has beenachieved, the isolating switch 25 is opened to disconnect the referencememory 24 from the subtracting 20, so that the memory 24 now stores afixed reference value of the resultant measurement signal correspondingto the desired set ventilating volume and delivers a signalcorresponding to this reference value to the input 27 of the comparator23. The current value memory 27 continues, however, to be updated withthe new value of the resultant measurement signal which is delivered atthe output of the subtracting circuit 20 at the end of each expirationphase of the breathing cycle and delivers a signal corresponding to theupdated current value to the input 26 of the comparator 23. If theventilating volume subsequently changes from the desired set value as aresult, for example, of changes in the compliance of the patient'slungs, the signals applied to inputs 22 and 26 of the comparator 23become different and the comparator delivers an error signal on output28 to the servo motor 29 which operates to adjust the control member 17in the required sense to restore the ventilating volume to its desiredpreset value.

Another embodiment of a medical respirator according to the presentinvention is illustrated diagrammatically in FIG. 2 of the drawings andcomprises a T-shaped ventilator duct 31 having a stem portion 32 coupledto the proximal end of a tracheal tube 33 for insertion in the airway ofa patient 34.

A first branch 35 of the ventilator duct 31 has its end connected to arespiratory fresh gas conduit 36 connected to a high pressure source 37of repiratory fresh gas. The respiratory fresh gas flowing through thefresh gas duct 36 is regulated to a relative low pressure by a flowregulator 38. The branch 35 of the ventilator duct is provided with aflow sensor 39 arranged to measure the volume of fresh gas suppliedthrough the branch 35 during the expiration phase of the breathingcycle.

The other branch 40 of the ventilator duct 31 has its end open andincorporates a flow sensor 41 arranged to measure the total volume ofgas passing through the branch 40 during the expiration phase of thebreathing cycle, this total expired volume being composed of the volumeof gas expelled from the lungs of the patient, i.e. the volume of gasventilating the patient's lungs, and the volume of fresh gas suppliedthrough branch 35 during the expiration phase.

A driving jet 42 is positioned in the ventilator duct 31 and is suppliedwith driving gas by a driving gas line 43 connected to the high pressuresource 37 of respiratory fresh gas. The driving gas supply line containsa solenoid valve 44 and a flow or pressure regulator 46 having a controlmember 47 which is manually settable and adjustable by a servomotor 59under the control of a control circuit which is similar to that of FIG.1 and comprises a subtracting circuit 50 receiving respectivemeasurement signals from flow sensors 39 and 41 on lines 48 and 49, areference memory 54 connected to output 51 of circuit 50 via isolatingswitch 55, a current value memory 57 connected permanently to output 51,comparator 53 having its output 58 connected to the servomotor 59, and adisplay device 60.

In the embodiment described with reference to FIG. 2, the low pressurerespiratory gas is supplied continuously through the conduit 36 duringoperation of the ventilator. The solenoid 44 delivers pulses ofrespiratory fresh gas to the jet 42 which thus emits a cyclicallyvarying flow of relatively high pressure respiratory gas which entrainsinto the stem 32 of the ventilator duct the lower pressure respiratorygas supplied through the branch 35 of the ventilator duct during theinspiration phase of breathing. During the expiration phase ofbreathing, the gases expelled from the patient's lungs are deliveredthrough the open branch 40 of the ventilator duct together with the lowpressure respiratory fresh gas which continues to flow through thebranch 35 of the ventilator duct.

In use of the FIG. 2 ventilator to maintain the volume of gasventilating from the patient's lungs at a desired preset level, thecontrol member 46 of the regulator 45 is initially adjusted until thedesired ventilating volume is achieved and the isolating switch 55 isthen opened so that the servomotor 59 is controlled in accordance withthe comparison of the reference value stored in reference memory 54 andthe updated current value stored in the current value memory 57 in orderto maintain the ventilating volume at a constant level.

Although the above embodiments of the invention have been described withreference to the maintenance of a desired value of the volume of gasventilating the lungs of the patient, it is envisaged that theventilator may be arranged to maintain any other appropriate parameterof the respiration process by providing suitable sensing means to detectthe current value of the parameter to be maintained during operation ofthe ventilator. For example, the concentration of carbon dioxide at theend of the expiration phase, i.e. the end tidal carbon dioxideconcentration, could be controlled to a desired value by providing asuitable carbon dioxide detector 70, such as a known infra red carbondioxide detector, to measure the carbon dioxide concentration in thevicinity of the proximal end of the tracheal tube 3 or 33 at therelevant instant in the breathing cycle. The electrical output from thecarbon dioxide detector could then be applied to the reference memoryand the current value memory in place of or instead of the output signalof the subtracting circuit 20 or 50.

We claim:
 1. A method of operating a medical ventilator to generate avariable pressure in the airways of a patient, with respiratory freshgas being continuously delivered during an expiration phase of eachbreathing cycle, comprising the steps of:initially setting the pressureto obtain a desired value of the volume of gas ventilating the lungs ofthe patient, which value is to be controlled; measuring the total volumeof expired gas flowing in the ventilator away from the patient duringthe expiration phase; measuring the volume of respiratory fresh gasdelivered by the ventilator during the expiration phase; subtracting themeasured volume of respiratory fresh gas from the measured total volumeof expired gas to calculate the volume of gas expelled from the lungs ofthe patient; detecting in the calculated value of the volume of gasventilating the lungs of the patient any change from the desired value;and adjusting the pressure generated by the ventilator in response tothe change in the value of the volume of gas ventilating the lungs ofthe patient so as to maintain the volume of gas ventilating the lungs ofthe patient at the desired value.
 2. A method according to claim 1wherein:the method further includes the step of measuring theconcentration of carbon dioxide in the total volume of expired gas; andwherein the adjusting step comprises adjusting the pressure generated bythe ventilator in response to the step of measuring the carbon dioxideconcentration.
 3. A medical ventilator, comprising:a controller forinitially setting the pressure of respiratory fresh gas, which iscontinuously supplied, to obtain a desired value of the volume of gasexpelled from the lungs of a patient during an expiration phase of abreathing cycle, which value is to be controlled; a measuring apparatusfor measuring the value of the volume of gas expired from the lungs ofthe patient during subsequent operation of the ventilator comprising:afirst measuring device arranged to measure the total volume of expiredgas flowing away from the patient in the ventilator during theexpiration phase, the first measuring device generating a firstmeasurement signal in response to the measured total volume; a secondmeasuring device arranged to measure the volume of respiratory fresh gasdelivered by the ventilator during the expiration phase, the secondmeasuring device generating a second measurement signal in response tothe measured volume of respiratory fresh gas; and a subtracting devicefor subtracting the second measurement signal from the first measurementsignal to generate a resultant measurement signal significant of thevolume of gas expelled from the lungs of the patient; a detectorresponsive to the resultant measurement signal for detecting any changein the measured value of the volume of gas expired from the lungs of thepatient from the desired value; and an adjustment device for adjustingthe controller in response to a detected change in the measured value ofthe volume of gas expired from the lungs of the patient in order to varythe pressure of respiratory fresh gas supplied by the ventilator so asto maintain the volume of gas expired from the lungs of the patient atthe desired value.
 4. A ventilator according to claim 3, furthercomprising:a reference memory to which the resultant measurement signalis delivered via a normally closed isolating switch which is opened uponsetting of the desired value of the volume of gas expired from the lungsof the patient to store an initial reference value of the resultantmeasurement signal in the reference memory; a current value memory whichis permanently connected to receive the resultant measurement signalfrom the subtracting device; a comparator for comparing the contents ofthe reference memory and the current value memory during operation ofthe ventilator and delivering an output error signal significant of anydifference between the contents of the reference memory and the currentvalue memory; and a servomotor connected to receive the error signalfrom the comparator and coupled to adjust the setting device of theventilator in response to the error signal.
 5. A ventilator according toclaim 3 wherein the setting device is arranged to control the supply ofdriving gas to a driving jet which is disposed in a ventilator duct ofthe ventilator and to determine the pressure of respiratory gas suppliedto the patient.
 6. A ventilator according to claim 5, wherein thesetting device is a flow regulator or a pressure regulator via which thehigh pressure driving gas is delivered to the driving jet.
 7. Aventilator according to claim 5, comprising the inlet at a patient endof the ventilator duct for connection to a source of respiratory freshgas, the driving jet being so spaced from the respiratory gas inlet thata column of gas in the respirator duct between the driving jet and theinlet acts as a pneumatic piston to transmit pressure from the drivinggas to the flow of respiratory fresh gas supplied to the patient end ofthe ventilator duct.
 8. A ventilator according to claim 5, wherein theventilator duct comprises a stem having a base which splits into firstand second branches, the base for connection to a patient, the firstbranch for connection to a source of respiratory fresh gas and thesecond branch through which expired gas flows during an expiration phaseof a breathing cycle, with the jet being disposed in the stem of theventilator duct so as to entrain respiratory fresh gas from the firstbranch of the ventilator duct during an inspiration phase of thebreathing cycle.
 9. A ventilator according to claim 4, further includinga measuring device for measuring the concentration of carbon dioxide inthe total volume of expired gas.
 10. A ventilator according to claim 3wherein:the setting device is arranged to control the supply of drivinggas to a driving jet which is disposed in a ventilator duct of theventilator and to determine the pressure of respiratory gas supplied tothe patient; the ventilator duct comprises a stem having a base whichsplits into first and second branches, the base for connection to apatient, the first branch for connection to a source of respiratoryfresh gas and the second branch through which expired gas flows duringan expiration phase of a breathing cycle, with the jet being disposed inthe stem of the ventilator duct so as to entrain respiratory fresh gasfrom the first branch of the ventilator duct during an inspiration phaseof the breathing cycle; the ventilator further comprises an inlet at apatient end of the ventilator duct for connection to a source ofrespiratory fresh gas; the driving jet is spaced from the respiratorygas inlet that a column of gas in the respirator duct between thedriving jet and the inlet acts as a pneumatic piston to transmitpressure from the driving gas to the flow of respiratory fresh gassupplied to the patient end of the ventilator duct; and the ventilatorfurther comprises a measuring device for measuring the concentration ofcarbon dioxide in the total volume of expired gas.